Fluoropolymers, are defined herein broadly as any of the fluorine containing polymers (or inert polymers), including homopolymers, copolymers, and terpolymers that have non-wettable and chemical inert surfaces which, although being desired in some applications, limit the use of these materials in other applications.
The technology of coating of articles with fluoropolymers has been developing along two fundamentally distinctive directions based on the physical form of powder and latex fluoropolymers. In each case, the final coating, which may be a continuous film layer, for example, is typically obtained by heating the applied fluoropolymers above their melting points.
Processes and products have been developed which provide specific advantages for powder and latex fluoropolymer applications. For technologies that use powdered fluoropolymers, modified polymer compositions and particle sizes and shapes have been developed to advance both the application yield (yield per pass) and the performance of the resulting film per unit film thickness. The major intrinsic obstacle to advancements in the use of powdered fluoropolymers is their poor electrical surface conductivity.
For latexes, the ultra low surface energy and the high specific gravity peculiar to fluoropolymers (they can be defined as being fully hydrophobic) has forced the adoption of different manufacturing technologies since the base polymer synthesis (e.g. dispersion) is characterized by polymer particles having an average diameter two orders of magnitude smaller than the powders, and by the extensive use of surfactants, both the fluorinated surfactants used during synthesis, and hydrogenated surfactants for the creaming of diluted dispersion obtained from the synthesis, and for the stabilization and formulation of concentrated latexes manageable by the application techniques (e.g. spray, roll, curtain coating). However, both kinds of surfactants, intrinsic to the technology, are detrimental to the coating application, negatively impacting the yield and the characteristics of the film layer (e.g. film continuity, adhesion to the substrate, etc.).
A way to escape from these two fundamental approaches is theoretically conceivable, and involves the modification of the fluoropolymer particle surface, to make it more compatible with the broad spectrum of available polar carrier means (e.g. water), but without altering/damaging the properties of the fluoropolymer bulk.
Surface treatments of fluoropolymer are known and established in the art. Fluoropolymers in the form of sheets, films and shaped articles have been chemically treated, subject to electrical discharged using corona discharge and plasmas, subject to flame treatment, and subject to physical treatment such as chemical adsorbing procedures. In each instance, desired results have often been less than satisfactory. For example, surface changes effected by chemical treatments produces darkening of the surface and chemical absorbing procedures are subject to deterioration and loss over time.
Flame treatments can cause undesired damage if not properly controlled.
Electrical treatments seem to have become the most accepted processes for desired long term effects. However, as discussed below, these treatment processes have limitations.
Corona discharge and flame treatment processes are used for treating the surfaces of polymer films and other substrates such as foils, papers, etc. These treatment processes increase the surface energy of the substrates, which in turn improves the wettability, printability and adhesion on these surfaces. Corona discharges can produce locally concentrated discharges known as streamers. These streamers lead to some non-uniformity in the treatment of the film surfaces, and the concentrated energy of the streamers can also microscopically damage the film surface. Furthermore, corona treatment can produce backside treatment, which is undesirable in many applications.
Flame treatment also has limitations in terms of oxidation surface modification, difficulty in control and possibility of excessive thermal loads.
Plasma treatment is an effective method for treating surfaces to increase surface energy and improve wettability, printability and adhesion. Plasma produces uniform surface treatment without causing backside treatment of the substrate.
Low-pressure or atmospheric plasma treatment (APT) processes have been developed that provide unique advantages over existing technologies for surface treatment. The apparatus used in atmospheric plasma treatment does not require a vacuum system, produces a high-density plasma and provides treatment of various substrates at low temperature while operating at atmospheric pressure. The benefits of plasma treatment include reduced degradation of surface morphology, higher treatment (dyne) levels, elimination of backside treatment, and extended life over treatment time.
As reported by A. Yializis et al. (Atmospheric Plasma—The New Functional Treatment for Film, 2000 TAPPI Polymers, Laminations, & Coatings Conference pp. 1343-1352), atmospheric plasma treatment processes have been developed for treating continuous webs and films.